如何实现 Flex 与 Webservice 之间的 XML 通信

Flex 代码

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">

<mx:Script>
<![CDATA[
    import mx.controls.Alert;

   function onClick():void{
    // 调用Webservice方法
    ws.HelloWorld.send();
   }
   function doneHelloWorld():void{
   }
  public var datas:XML =<user>
<menuName>burger</menuName>
<price>3.95</price>
</user>
]]>
</mx:Script>

<mx:WebService id="ws" wsdl="http://localhost/InfoManage/webservice/WebService.asmx?wsdl"
  showBusyCursor="true" useProxy="false">
  
  <mx:operation name="HelloWorld" result="doneHelloWorld">
   <mx:request>
       
    <username><![CDATA[{datas.toXMLString()}]]></username>
   </mx:request>
  </mx:operation>
</mx:WebService>
<mx:Button x="347" y="138" label="Button" click="onClick();"/>
<mx:TextInput id="ti" x="299" y="97"/>
  <mx:Label id="la" x="362" y="187" text="{ws.HelloWorld.lastResult}"/>
</mx:Application>

Webservice 代码
using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Text;
using System.Xml;

/// <summary>
/// WebService 的摘要说明
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class WebService : System.Web.Services.WebService {

    public WebService () {

        //如果使用设计的组件，请取消注释以下行
        //InitializeComponent();
    }

    [WebMethod]
    public string HelloWorld(string username) {
        XmlDocument doc = new XmlDocument();
        return username;
    }

}

结果只能显示<user> 我想显示所传递进的XML文档格式的字符串....
如何在Webservice内获取传入信息...请各位帮忙...

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补充：.NET技术 ,  Web Services